Updating as to not leaving a hanging thread.
-HDR you would have been more than welcome to that huge slab of metal, but Indiana is a long way.
Once the plate was removed the reason behind it was evident. The three center sections had dips between the floor supports. Looks like the Gemini adVantage did some heavy hauling in its previous life. I was unable to see the dips from the underside due to obstacles (fuel tank, exhaust heat sheilds...) obscuring the view.
Ended up acquiring a thinner sheet of steel (junkyard), and shed about 1/2 the weight of the original plate that was previously installed. Still a bummer lugging the extra weight, but content with that aspect of the build completed. Thanks for all the responses to my post.